This invention relates to capacitive transducers, for example for use in displacement and force-responsive devices.
Capacitive displacement transducers are known for use in displacement and force-responsive devices such as measurement probes and joysticks, where a stylus or lever is movable in the directions of two or more orthogonal axes. Normally there would be one or more separate capacitive transducers for each axis of movement, as shown in U.S. Pat. No. 5,661,235. This not only results in a costly, difficult to assemble structure, but also degrades the performance when measuring extremely small forces and displacements, due to the large moving mass which limits the measurement frequency response and increases the sensitivity to external vibrations. Thomas (U.S. Pat. No. 5,006,952) teaches a multiple axis capacitive displacement transducer that uses a single movable pick-up plate. This device does not have the difficult assembly and large moving mass problems as the individual transducer devices have, but it has two other drawbacks, when applied to high precision measurements, that are solved by the present invention. The first drawback is that the pick-up plate is supported, and pivots about a point on the stylus or stem some distance away from the pickup plate. This causes an undesired translational motion in addition to the desired rotational response for x and y axis displacement of the stem. This undesired translational response may generate undesired cross-axis readings. For instance, a pure x-axis displacement of the stem may produce a false Z-axis reading. The other problem is that the signal channels are separated from each other by operating at different frequencies. This results in the individual channels having different noise and frequency response characteristics, which is generally undesirable for precision measurements.
Precision capacitive displacement transducers typically employ three electrodes, which form a structure equivalent to two capacitors connected in series, with the center electrode being movable and common to both capacitors. The center electrode is also typically the pickup electrode, and the two outer electrodes are mechanically fixed. Although the transducer is fundamentally responsive to displacement of the center electrode, it can be used to measure force, by the deflection of springs of known stiffness in response to that force, as well as acceleration or pressure. Bonin et al. (U.S. Pat. No. 4,694,687) discloses a vehicle performance analyzer which incorporates a capacitive accelerometer based on the three electrode structure described here. By driving the outer electrodes with two equal amplitude signals 180 degrees out of phase, the voltage on the center electrode is a linear function of the displacement from the center, with the phase giving polarity information. The full scale amplitude of the output signal is equal to the amplitude of the drive signal. This is a great improvement over strain gauge type load cells which are also used to measure displacement and force. Strain gauges typically have a full scale output signal that is 0.2% of the input signal, giving the capacitive transducer 500 times greater output signal.
By synchronously demodulating the center electrode signal of the capacitive transducer, a DC voltage proportional to the displacement is generated. In the absence of parasitic effects such as amplifier input capacitance, the output signal of voltage vs. displacement would be perfectly linear, but for conveniently scaled devices, the transducer source capacitance may be on the order of 5 pF and the parasitic capacitance may be 1 pF or more, so the non-linearity is significant, on the order of 20% at full scale in this case. For multiple axis devices sharing a common center electrode, the parasitic capacitance may actually be greater than the sense capacitance per axis. It is possible to eliminate the effect of this parasitic capacitance on the signal linearity by generating a feedback signal that is added to one drive signal and subtracted from the other, in order to maintain a null, or zero voltage situation on the center electrode regardless of displacement. In this case the feedback signal is used as the output signal, and is proportional to the center electrode displacement regardless of parasitic capacitance. Thomas uses this feedback method, and also references British Patent No. GB 1366284.
One application requiring measurement of force and possibly displacement in at least two directions is scratch testing of materials to determine coating adhesion and resistance to wear. In this test, a series of passes at increasing loads are made over the material with a stylus, until reaching a load that causes delamination or other catastrophic failure. Typically, both the vertical and horizontal load forces are recorded vs the horizontal position. The vertical displacement of the stylus into the sample may also be recorded. A more sophisticated test uses a ramped vertical load to get the same information from a single, rather than a series of scratches. This measurement requires that there be very little interaction between the signals of the different axes, so that the coefficient of friction, that is the horizontal force divided by the lateral force, can be accurately determined. A similar, more specific application involves tribological studies of materials for rigid disc drive applications, such as measuring the friction properties of various slider materials on a disc surface. In both cases, a low moving mass is desirable to allow a higher measurement bandwidth than is possible with prior art devices such as described in U.S. Pat. No. 5,661,235.
Another application requiring high measurement sensitivity due to the small size of the devices being measured is in the mechanical testing of MEMS devices. These devices are typically 100 to 1000 microns in length, and may have elements with dimensions as small as 1 micron in width or thickness. Due to the very small size of the parts, force sensitivity of one micro newton or better is desired. Multiple axis capability is desired so that measurements can be made in any direction required by the sample, although each measurement is typically made in a single direction.
The present invention provides a force or displacement transducer operative in at least two nominally orthogonal directions in a first embodiment. In a second embodiment, the transducer of the first embodiment is incorporated into an apparatus for measuring mechanical properties of MEMS devices or other small devices, hardness and scratch resistance of thin films and surfaces, and friction and wear properties of small components such as sliders used in the disc drive industry.
The transducer consists of a centrally located plate shaped pickup electrode, also referred to as the center electrode or as the pickup plate and several pairs of drive plates on opposing sides of the pickup plate. Preferably there are four pair of drive plates. The drive plates are of a conductive material, which may be copper, fabricated on an insulating substrate for mechanical support using techniques well known in the printed circuit board industry. The center electrode is also formed of a conductive material, but preferably of higher strength than pure copper, such as a high strength Beryllium copper alloy. Support springs for maintaining the proper position of the center electrode are formed integrally with the center electrode by photochemical etching which is a well known process. By arranging the support springs to connect to the center electrode in the same plane as the center electrode, rather than connect to the load stem at some point away from the center electrode, undesired lateral motion of the center electrode which could generate erroneous cross axis signals is eliminated. This also allows for more convenient electrical connection to the moving center electrode, by using the fixed end of one or more of the springs as the electrical contact.
At least one of the outer electrode support substrates contains a central hole through which a stem or probe tip is attached to the center electrode. If this stem is fabricated of an electrically insulating material it may be fastened directly to the center electrode by adhesive, or by a small screw, or any other suitable means. If the stem is fabricated out of a conductive material such as metal, an insulating bushing may be included between the stem and the center electrode to avoid electrical interference being conducted directly to the center electrode. The springs allow the center electrode to deflect in response to forces applied to the probe tip. A force applied directly in line with the probe tip (along the Z-axis) causes the entire center electrode to move closer to the drive plates on one side, and farther away from the drive plates on the other side, while maintaining a parallel relationship between the center electrode and the drive plates. A force applied in a horizontal direction (in the X or Y axis) causes or tilting or rotation of the probe, and the center electrode attached to it.
The position of the center electrode is determined by measuring signals induced on the center electrode by the drive plates. The closer the center electrode is to a drive plate, the more signal it picks up from that drive plate. In that manner a displacement of the drive plate from its normal center position is detected, and the force responsible for that displacement is determined by multiplying the stiffness of the support springs by the measured displacement. The displacement of the portion of the center electrode between each of the four pair of drive plates is determined by applying a pulse to each drive plate pair in sequence, and synchronously measuring the drive plate response signal with the appropriate sense channel, there being one channel for each pair of drive plates. The X, Y and Z axis force components are determined simultaneously by combining the appropriate channels. The Z-axis force is determined by adding all four channels, the Y-axis force is given from the difference between the two Y-axis channels and the X-axis force is determined by the difference between the two X-axis channels.
Determining the relative position of the four channels in this manner, and then combining those signals to get the three axis force information has several advantages over the prior art as disclosed by Thomas. The first advantage is that the electrical frequency response of all three axes are the same, which is desirable for precision measurement instruments, whereas there is a 4:1 difference between the axes in the Thomas method. Also the noise is lower and more consistent between the channels in the current invention, as all channels operate at the same frequency. Whereas, when operating at different frequencies, the channel operating at the lowest frequency will have the greatest noise, since the impedance of a capacitive device is greater at lower frequencies, and greater impedance results in greater noise. Noise is also reduced in the current invention by avoiding the separate Z-axis electrodes, so that for a given transducer size, the active electrode area is greater, which further reduces the impedance. A final benefit of the current invention is that of improved signal linearity. In capacitive transducers as described here and in the prior art, the linearity of the signal is decreased when the electrode plates are not parallel to each other. In both the current invention, and in Thomas, X or Y axis forces cause the center electrode to rotate out of parallel, but the effect on linearity is less severe in the current invention, since the Z-axis ring electrode of Thomas is eliminated. By eliminating the ring, and using only the four quadrant electrodes, the effect of the center electrode tilt is reduced, as the active drive plate covers only about ⅓ of the width of the center electrode, reducing the effect of the tilt on the linearity by the same factor.
The multi-dimensional capacitive transducer has many applications, including use in an instrument for tribological property testing, and also mechanical testing of small structures such as MEMS devices. The tribological property tester consists of a mechanism to provide relative motion between two samples being tested, with one of the samples being mounted on the multi-dimensional capacitive transducer. This provides reading of both the normal (Z-axis) load force and the resultant frictional force, so that friction coefficient vs load force, time, speed and other parameters can be readily determined. For testing MEMS devices, the device to be tested is mounted on a moveable stage, which could be a stage on an optical microscope. The multi-dimensional capacitive transducer is mounted above the device to be tested. If the stage just referred to is on an optical microscope, the transducer may be mounted in place of one of the optical objectives. This allows the device to be tested to be optically inspected and positioned, so that when the turret is rotated to engage the multi-dimensional capacitive transducer, the probe stem mounted on the transducer will be in the proper location. The microscope""s focusing mechanism is used to provide Z-axis motion, and the microscope stage provides the x-y motion. Due to a lack of commercially available instruments of adequate sensitivity, researchers fabricating and testing MEMS devices often build into the device some sort of structure for performing mechanical testing of the device. Unfortunately, due to the small size of these devices, it is difficult if not impossible to accurately calibrate these test structures. This had led to the publication of some reputed mechanical properties variations that are more likely due to measurement error than any real physical effect. Due to the high sensitivity and low moving mass of this device, it can measure forces down to 1 micro newton or less, yet still be calibrated accurately using conventional methods such as scale calibration weights.
These and various other advantages and features of novelty which characterize the present invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the object obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter in which there are illustrated and described preferred embodiments of the present invention.